Refactor newfile() / newdir() into primarily in-place APIs. A
temporary directory can still be created with newdir().
Before this refactor the API looked slightly nicer, but we would never
be able to reach the efficiency of the native file APIs. In
particular, we might end up moving data across devices as a last step
in constructing a directory tree — this seemed bad!

Various changes to BlobTree API to make it more Dict-like and less
reliant on overloading filesystem-like functions.

* Further document the BlobTree API
* Allow path strings as keys in more places in BlobTree
* Deprecations of some filesystem-like functions

* The big rename
* Add deprecations for Blob, BlobTree

Not including documentation on writing to a file tree yet; that will
come in a future change.

Also some docstring refinement

* Additional tests for FileTree - the whole API should be covered now.
* Remove internal TempFilesystemRoot; replace with a simple flag on
  FileSystemRoot.
* Remove internal AbstractFileSystemRoot; this seems unnecessary now.